Functional-Group Organic Chemistry and Biomolecules — Practice

Question 1

Which halide reacts fastest by the SN1 mechanism?
(a) (CH₃)₃C–Br (b) CH₃–Br (c) CH₃CH₂–Br (d) CH₃CH₂CH₂–Br

Accepted Answer

(a) (CH₃)₃C–Br.

SN1 goes through a carbocation, and the tertiary carbocation is the most stable, so the tertiary halide reacts fastest by SN1 (primary/methyl halides prefer SN2).

Question 2

The SN2 reaction proceeds with:
(a) inversion of configuration (b) retention of configuration (c) racemisation (d) no stereochemical change

Accepted Answer

(a) inversion of configuration.

The nucleophile attacks from the side opposite the leaving group, flipping the configuration (Walden inversion). SN1, going through a planar carbocation, gives racemisation.

Question 3

The correct order of acidic strength is:
(a) CH₃COOH > C₆H₅OH > H₂O > C₂H₅OH (b) C₂H₅OH > H₂O > C₆H₅OH > CH₃COOH (c) C₆H₅OH > CH₃COOH > C₂H₅OH > H₂O (d) H₂O > CH₃COOH > C₆H₅OH > C₂H₅OH

Accepted Answer

(a) CH₃COOH > C₆H₅OH > H₂O > C₂H₅OH.

Carboxylate delocalises charge over two oxygens (most acidic), phenoxide over the ring, water is neutral reference, and alkoxide has no stabilisation (least acidic).

Question 4

Which is the strongest acid?
(a) Cl₂CHCOOH (b) ClCH₂COOH (c) CH₃COOH (d) HCOOH

Accepted Answer

(a) Cl₂CHCOOH.

More electron-withdrawing chlorine atoms stabilise the carboxylate anion by the −I effect, so dichloroacetic acid is the strongest of these acids.

Question 5

Tollens' reagent gives a silver mirror with:
(a) aldehydes (b) ketones (c) alcohols (d) ethers

Accepted Answer

(a) aldehydes.

Aldehydes reduce the Ag⁺ in ammoniacal silver nitrate to metallic silver (a mirror). Ketones do not react, so Tollens' distinguishes aldehydes from ketones.

Question 6

A positive iodoform test (yellow CHI₃) is given by:
(a) acetone (b) benzaldehyde (c) formaldehyde (d) diethyl ether

Accepted Answer

(a) acetone.

The iodoform test is positive for methyl ketones (CH₃CO–) and for ethanol/acetaldehyde (CH₃CH(OH)–). Acetone (CH₃COCH₃) qualifies; benzaldehyde and formaldehyde do not.

Question 7

Aldehydes are more reactive than ketones toward nucleophilic addition because ketones have:
(a) more steric hindrance and greater +I electron donation (b) a stronger C=O bond (c) no carbonyl group (d) higher molecular mass

Accepted Answer

(a) more steric hindrance and greater +I electron donation.

Two alkyl groups in a ketone crowd the carbonyl carbon and donate electron density (+I), reducing the partial positive charge, so the nucleophile adds less readily than to an aldehyde.

Question 8

In aqueous solution, the correct order of basicity is generally:
(a) 2° > 1° > 3° > NH₃ (b) 3° > 2° > 1° > NH₃ (c) NH₃ > 1° > 2° > 3° (d) 1° > 2° > 3° > NH₃

Accepted Answer

(a) 2° > 1° > 3° > NH₃.

In water, solvation of the protonated ammonium ion competes with the +I effect, giving the observed order 2° > 1° > 3° > NH₃ for simple alkyl amines.

Question 9

Aniline is a weaker base than methylamine because:
(a) the nitrogen lone pair is delocalised into the ring (b) it has no lone pair (c) it is more soluble (d) the ring is electron-withdrawing by hyperconjugation

Accepted Answer

(a) the nitrogen lone pair is delocalised into the ring.

In aniline the N lone pair is drawn into the benzene ring by resonance, so it is less available to accept a proton, making aniline a much weaker base than aliphatic amines.

Question 10

The carbylamine (isocyanide) test is used to detect:
(a) primary amines (b) secondary amines (c) tertiary amines (d) alcohols

Accepted Answer

(a) primary amines.

Primary amines react with chloroform and alcoholic KOH to give foul-smelling isocyanides (carbylamines); secondary and tertiary amines do not, so the test is specific for primary amines.

Question 11

Which of the following is a non-reducing sugar?
(a) sucrose (b) glucose (c) maltose (d) lactose

Accepted Answer

(a) sucrose.

In sucrose both anomeric carbons are tied up in the glycosidic bond, leaving no free aldehyde/ketone group, so it does not reduce Tollens'/Fehling's. Glucose, maltose and lactose are reducing.

Question 12

The α-helix and β-pleated sheet of a protein are stabilised mainly by:
(a) hydrogen bonds (b) peptide bonds (c) disulphide bridges (d) ionic bonds

Accepted Answer

(a) hydrogen bonds.

Secondary structure (α-helix, β-sheet) is held by hydrogen bonds between backbone C=O and N–H groups. Peptide bonds define the primary sequence; denaturation breaks the hydrogen bonds.

Question 13

The linkage between two amino acids in a protein is a:
(a) peptide (amide) bond (b) glycosidic bond (c) ester bond (d) phosphodiester bond

Accepted Answer

(a) peptide (amide) bond.

The –COOH of one amino acid condenses with the –NH₂ of the next to form a –CO–NH– peptide (amide) bond, releasing water.

Question 14

In DNA, adenine pairs with:
(a) thymine (b) uracil (c) guanine (d) cytosine

Accepted Answer

(a) thymine.

Adenine pairs with thymine (two hydrogen bonds) and guanine with cytosine (three) in DNA. In RNA, adenine pairs with uracil instead of thymine.

Question 15

Deficiency of vitamin C causes:
(a) scurvy (b) rickets (c) night blindness (d) beri-beri

Accepted Answer

(a) scurvy.

Vitamin C (ascorbic acid) deficiency causes scurvy. Vitamin D deficiency causes rickets, vitamin A night blindness and vitamin B₁ beri-beri.

Practice — Functional-Group Organic Chemistry and Biomolecules

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